The present invention relates to an improved apparatus and method for controlling a centerless grinding machine to more accurately control the final configuration of objects having a cylindrical cross-section. The invention is particularly useful in situations where the final configuration of the ground object is characterized by fixed lengths of varying diameters and/or tapered sections. The invention has particular application to grinding guide wires for guiding catheters during angioplastic procedures or other procedures requiring catheterization, but may be applied to any product requiring varying fixed diameters or tapers, such as golf club shafts, arrows, whip antennae and fishing rods, which may be up to ten or more feet in length.
Generally, centerless grinders are used to grind the outer surface of a rod or wire. The object of the grinding operation is to produce a wire that is round and that has a diameter and surface finish in accordance with given specifications at any given cross-section along its length.
Typically, a wire is fed into a centerless grinder at one end and guided between two grinding wheels that rotate in the same direction at different speeds, known as the work wheel and the regulating wheel. The wire rotates as a result of its contact with the regulating wheel and is ground to a specified diameter dictated by the distance between the faces of the two grinding wheels. One of the grinding wheels, typically the regulating wheel, can be moved so that the distance between the faces of the grinding wheels may be varied during the grinding process.
The wire advances through the grinding machine as a result of its contact with the grinding wheels. Specifically, one of the grinding wheels, typically the regulating wheel, rotates along an axis that is almost parallel to the axis of rotation of the wire being ground, but slightly skewed in a vertical plane, so that its contact with the wire causes the wire to move forward through the machine. The angle at which the axis is skewed is commonly referred to as the tilt angle and generally varies between one and three degrees.
A number of factors can affect the rate at which the wire moves through the grinding machine. For example, temperature, regulating wheel RPM, regulating wheel tilt angle, slippage, type of coolant used, wire diameter, wire material, wire material uniformity, and grinding wheel material may affect feed rate. Thus, the feed rate cannot accurately be controlled and often varies substantially during the grinding process.
The prior art methods of producing wire of multiple fixed diameters and tapers are deficient because they do not account for varying feed rates. For illustrative purposes it is assumed that it is desired to produce a wire as depicted in FIG. 1 having a given fixed diameter section 10 for a first unit of length 17, a tapered section 11 for a second unit of length 16, and a second fixed diameter section 12 for a third unit of length 15.
In a prior art method of grinding such a wire the work wheel is dressed, i.e., formed, so that it will produce one tapered section and one fixed diameter section on a wire in any one given pass. For example, referring to FIG. 1, it can produce fixed diameter section 10 and tapered section 11 in one pass. More particularly, the grinding face of the work wheel is configured so that as the wire moves through the space between the work wheel and the regulating wheel the gap becomes more narrow until a certain point. At that point, the face of the work wheel becomes parallel to its axis of rotation so that the size of the gap between the wheels remains the same.
In operation, the wire is fed into the machine and allowed to progress until the point of transition between the parallel and tapered sections on the work wheel matches the point along the length of the wire where the taper is to begin. The wire is then withdrawn and the regulating wheel position is adjusted to grind the second fixed diameter and tapered section of the rod. If the taper angle of the second tapered section does not match the first taper angle, an entirely new work wheel with a different dress must be used. Thus, this prior art method is cumbersome and tedious. In addition, if the operator does not stop the grinding process at precisely the correct point, which can be difficult, the length of the fixed diameter sections of the wire will not meet specifications.
In another prior art method the wire is placed on an elongated feed bed and moves longitudinally along the feed bed as it is being ground. Sensors which sense the passage of the trailing end of the wire are mounted along the feed bed so that the end of the wire passes the sensors in succession. The position of the sensors are adjustable, and the operator positions the sensors on the feed bed so that when the end of the wire passes a sensor, that event corresponds to a point in the grinding process at which a transition from a fixed diameter section to a tapered section, or vice versa, should occur. The sensors generate signals and the commencement and cessation of movement of the regulating wheel is keyed to those signals. To produce a tapered section in this prior art method, when a signal is generated by one of the sensors indicating that a taper is to begin, the regulating wheel is moved away from the work wheel at a fixed constant rate while the wire continues to be fed into the grinding machine. The rate at which the regulating wheel is moved away from the work wheel is based on an assumed constant wire feed rate. However, since the actual feed rate may vary substantially, the produced wire taper may be of irregular shape and fall outside of specified limits. This prior art method is deficient because movement of the regulating wheel is not related to the actual position of the wire or rod.